A. I. Kiselev

Middle Paleozoic basic magmatism of the northwestern Vilyui Rift: Composition, sources, and geodynamics

Middle Paleozoic magmatism at the eastern Siberian platform was related to riftogenic processes, which were most clearly expressed in the Vilyui Rift and led to the formation of rift depressions filled with sedimentary-volcanogenic rocks and extended basaltic dike belts in rift shoulders. Two fields of diamondiferous kimberlites were found along with basaltic dikes in the Vilyui-Markha dike belt surrounding rift in the northwest. Active subalkali basaltic magmatism predated the emplacement of kimberlite bodies, which occasionally (Nyurba pipe) are cut by dikes of potassium alkali basalts. Based on geochemical and Sr-Nd isotopic characteristics, deep-seated sources were determined for the intrusive and volcanic basalts of the northwestern shoulder of the Vilyui rift. The REE distribution patterns of the studied rocks normalized to the primitive mantle are close to that of OIB, except for somewhat higher HREE. In the diagrams of indicator ratios of trace and rare-earth elements, the basalts are also plotted in the OIB field, being located between the end member of plume composition (FOZO) and enriched mantle sources. The rocks have positive εSr (+3.5 and +28.6) and εNd (+1.3 and +5.3). In a diagram εNd(T)-εSr(T), two fields with distinct content of radiogenic Sr are distinguished, which can be regarded as derived by mixing of the moderately depleted PREMA-type mantle and a source enriched in radiogenic Sr. Available isotope-geochemical data confirm that OIB type basalts of the region were generated by plume activity. The geodynamic setting of Middle Paleozoic magmatism and rifting in the eastern part of the Siberian platform is considered in light of plume-lithosphere interaction. The sequence of tectonomagmatic events during evolution of the Vilyui rift is consistent with the model of plume-lithosphere interaction or the model of active rifting.

Devonian Swarms of Dolerite Dykes in the Northeastern Siberian Craton and Their Relation to the Vilyui Plume

Recently, many researchers have started to pay attention to the study of large dike belts. This is explained by the fact that such belts mark extended faults in the lithosphere and, thus, both play an indic� ative role in defining epochs of important lithospheric reorganizations and serve as reference points in paleo� geodynamic reconstructions, which serve as a basis for recent metallogenic and prognostic concepts and reconstructing past deepseated geodynamic pro� cesses. Like many others structures, the Siberian Craton was subjected to many reorganizations during its life. Its lithosphere was stabilized in the Paleoproterozoic approximately 1.9 Ga ago (1). During its subsequent history, the craton was repeatedly involved in different continental agglomerations (for example, during the formation of Rodinia) and then separated from them during destruction of supercontinents. The corre� sponding events are reflected in differentage dike swarms. The Late Riphean dike belts that appeared during the breakup of Rodinia and indicate at least a Late Riphean age of the southern boundary of the cra� ton are best known among them. Significant swarms and clusters of dykes were also formed in the Middle Paleozoic. Powerful eruptions of Siberian Trap with the formation of a spacious system of sills and dykes occurred at the Permian-Triassic transition, and kim� berlites of a new generation intruded the northeastern part of the craton in the Late Mesozoic One of the most impressive systems of dike swarms was formed in the eastern part of the Siberian Craton during the Devonian. Its development accompanied the formation of the Vilyui rift system. In the opinion of (2), this process crowned the breakup of the conti� nent and formed its eastern boundary. An important feature of this rift system is the radial distribution of its main structural elements, dike swarms included. Such a position and structural features of the rift system allowed its formation to be related to destructive pro� cesses above the mantle plume (3), the center of which was located in the area overlain in the presentday structure by the foreland of the Mesozoic Verkhoyansk fold-thrust belt. In compliance with data on the distribution of gra� bens and dike swarms, the plume head at the base of the rift system should be 1500 km across. The follow� ing questions naturally arise: what mantle substrate was involved in the plume formation and to what extent was the latter uniform in different areas of the giant asthenospheric lens above the plume? Our previ� ous studies of igneous rocks, dykes included, in the southwestern part of the Vilyui rift system (Vilyui rift proper) revealed the main geochemical and isotopic- geochemical parameters of their mantle sources (4).

Potassium basalts and picrobasalts from the devonian kimberlite fields of Western Yakutia, Russia, and their relations to kimberlite magmatism

Basic explosion pipes occur along with basic dikes, sills, and chonoliths within the Vilyui—Markha basic dike belt in the northwestern marginal part of the Vilyui Rift, characterized by widespread basaltic magmatism. The explosion pipes are of interest for exploration geology owing to their specific composition and tectonic setting, similar in many respects to the structural localization of kimberlite bodies in the sedimentary cover of the Siberian Platform. The basic explosion pipes from the Mirny district were referred to as tholeiitic and alkali-basaltic petrochemical rock series. Peculiar potassic and ultrapotassic rocks—potassium olivine basalts and picrobasalts—were identified in the alkali-basaltic series. These rocks were regarded as related to the deepest sources among basalts and were recommended for use as a prospecting guide for primary diamond sources. Our investigations allowed us to interpret the elevated K and Mg contents in basic fragments from some explosion pipes and associated intrusive bodies as a result of low-temperature metasomatic alteration. The explosion breccias and metasomatically altered basic rocks probably mark areas favorable for explosion activity and intrusion of both basic and kimberlitic rocks.

The northeastern boundary of the Siberian Craton and its formation peculiarities (derived from occurrences of early Cambrian and Devonian intraplate magmatism)

1252 The formation of the Siberian Craton ended in the Paleoproterozoic. Subsequently, it was repeatedly sub jected to destruction in response to interaction between the lithosphere and the mantle plumes. The most significant events of that time were the Neoprot erozoic and Middle Paleozoic rifting epochs, which determined the configuration of the Siberian Craton close to the present one. For example, the Neoprot erozoic rifting epoch is correlated with the breakup of Rodinia [1]. This epoch culminated with separation of the craton from the Laurentian part of Rodinia in the late Neoproterozoic [2] and formation of its southern (in modern coordinates) boundary. The Middle Pale ozoic rifting epoch resulted in the formation of the eastern boundary of the Siberian Craton [3]. The con tinental breakup was accompanied by the formation of a triple rift system, the best known branch of which is represented by the intracontinental Vilyui rift pinch ing out in the craton body [4]. Two other branches of this system determined the breakup boundary of the continent and separation of its eastern segments. These branches are represented by the Verkhoyansk and Sette Daban rifts, fragments of which are now observable in the Olenek (Kharaulakh segment) and South Verkhoyansk marginal zones of the Mesozoic Verkhoyansk fold–thrust belt (VFTB) [5].

Comparative characteristics of Middle Paleozoic and Late Riphean plume magmatism of the Siberian platform

Based on isotopic and geochemical data for Late Riphean dikes and sills and for Devonian dolerite dikes and basalt covers within the Sette-Daban rift in the western part of the Siberian platform, we proved conceptions about the participation of various deep sources in their formation. The inverse correlation in Devonian basites between concentrations of Nb, light rare earth elements, and a number of other highly incompatible elements on the one hand and Zr, Y, and other moderate incompatible elements, including heavy rare earth elements on the other hand allows us to assume that two sources participated in the formation of melts. The source of dolerites is close to the EMORB type, and the source of basalts is close to the OIB type. The compositions of Riphean rocks correspond to a trend for which magma formation occurred with the participation of a source with characteristics between NMORB and EMORB and also a component typical of subduction zones. The data obtained imply associate formation of basites with the influence of mantle plumes on the lithosphere of the southeastern part of the Siberian craton in the Late Riphean and Middle Paleozoic. In the Riphean the plume mantle was composed of a moderately depleted mantle of the EMODB type and a mantle with the composition close to the above-subduction mantle, which was metosomatically changed under the influence of fluid water, which caused the appearance of a Nb and Ta deficit in melting products. The isotopic characteristics Nd(147Sm/144Nd = 0.165 and ɛNd(T) ∼ 2.3–4.7) of rocks show the moderate depleted nature of these sources.

The Chara–Sina dyke swarm in the structure of the Middle Paleozoic Vilyui rift system (Siberian Craton)

The formation of the Vilyui rift system in the eastern Siberian Craton was finished with breakdown of the continent and formation of its eastern margin. A characteristic feature of this rift system is the radial distribution of dyke swarms of basic rocks. This peculiarity allows us to relate it to the breaking processes above the mantle plume, the center of which was located in the region overlain in the modern structure by the foreland of the Verkhoyan folded–thrust belt. The Chara–Sina dyke swarm is the southern part of a large area of Middle Paleozoic basaltic magmatism in the eastern Siberian Craton. The OIB-like geochemical characteristics of dolerite allow us to suggest that the melting substrate for Middle Paleozoic basaltic magmatism was represented by a relatively homogeneous, mid-depleted mantle of the plume with geochemical parameters similar to those of OIB.

Early Cambrian magmatism in the northeastern Siberian Craton (Olenek Uplift)

The Vendian–Lower Cambrian tectonomagmatic activation took place in the northeastern Siberian Craton, within the Olenek Uplift and in the Kharaulakh segment of the Verkhoyansk fold-and-thrust belt (the lower reaches of the Lena River). The Early Paleozoic volcanic activity in the Olenek Uplift is expressed in the form of basitic diatremes, small basaltic covers, and doleritic dikes and sills intruding and covering the Upper Vendian carbonate deposits. The material specificity of the Lower Cambrian basites and their mantle sources, jointly with the Vendian–Cambrian sedimentation history, gives reason to consider the Lower Cambrian riftogenesis and the associated magmatism as a consequence of the plume–lithosphere interaction in the northeastern Siberian Craton.